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of low power-consuming devices and sys-tems in self-powered mode without any charging of batteries, the development of PNGs with large power-generating perfor-mance, high sensitivity, and prominent mechanical durability is extremely impor-tant. Up to now, various kinds of PNGs applications have been demonstrated, including self-powered photodetectors, gas sensors, tactile sensors, powering com-mercial light emitting devices (LEDs), and liquid crystal displays (LCDs). [ 9–14 ]

Recently, various types of PNGs made by inorganic semiconducting piezoelec-tric ZnO nanowires, GaN nanowires, and lead-based and lead-free perovskite mate-rials with large piezoelectric coeffi cients such as Pb(Zr,Ti)O 3 [PZT], NaNbO 3 ,

KNbO 3 , BaTiO 3 , and ZnSnO 3 nanostructures have been suc-cessfully fabricated. [ 15–21 ] Polyvinylidene difl uoride (PVDF) as a piezoelectric polymer has especially attracted much attention in order to fabricate transparent and mechanically durable PNGs with a simple, reliable, and cost effective way. [ 22,23 ] It was dem-onstrated that the poly(vinylidenefl uoride- co -trifl uoroethylene) [P(VDF-TrFE)]-based PNGs could offer multifunctionality such as high sensitivity, fl exibility, stretchability, and multishape transformability. [ 24–29 ] For stable and continuous self-powered operation of devices and systems, large power output from the PNGs is greatly desirable. In this regard, various types of composite structures such as NaNbO 3 nanoparticles/PVDF and multiwall carbon nanotubes/PVDF were utilized to enhance the electric power output from the PVDF-based PNGs and improve the power generation effi ciency of PVDF-based PNGs. [ 30,31 ] Although the electric power output from the PVDF composite-based PNGs is increased compared to that from the pristine PVDF-based PNGs, the fabrication of PVDF composite-based PNGs still faces challenges such as complex fabrication pro-cess, noncost effectiveness, less reproducibility, nonuniform power output, diffi culty of interface control between inorganic nanostructures and the PVDF polymer matrix. The develop-ment of simple and low cost approach for improving electric power output from PVDF-based PNGs without forming com-plex composite structures is promising to realize various self-powered device applications.

In this work, we demonstrate a trigonal line-shaped and pyramid-shaped micropatterned P(VDF-TrFE) polymers-based PNGs. The output voltage and current from both

Micropatterned P(VDF-TrFE) Film-Based Piezoelectric Nanogenerators for Highly Sensitive Self-Powered Pressure Sensors Ju-Hyuck Lee , Hong-Joon Yoon , Tae Yun Kim , Manoj Kumar Gupta , Jeong Hwan Lee , Wanchul Seung , Hanjun Ryu , and Sang-Woo Kim *

Here micropatterned poly(vinylidenefl uoride- co -trifl uoroethylene) (P(VDF-TrFE)) fi lms-based piezoelectric nanogenerators (PNGs) with high power-generating performance for highly sensitive self-powered pressure sensors are demonstrated. The microstructured P(VDF-TrFE)-based PNGs reveal nearly fi ve times larger power output compared to a fl at fi lm-based PNG. The micropatterning of P(VDF-TrFE) polymer makes itself ultrasensitive in response to mechanical deformation. The application is demonstrated successfully as self-powered pressure sensors in which mechanical energy comes from water droplet and wind. The mechanism of the high performance is intensively discussed and illustrated in terms of strain developed in the fl at and micropatterned P(VDF-TrFE) fi lms. The impact derived from the pat-terning on the output performance is studied in term of effective pressure using COMSOL multiphysics software.

DOI: 10.1002/adfm.201500856

J.-H. Lee, T. Y. Kim, Prof. S.-W. Kim SKKU Advanced Institute of Nanotechnology (SAINT) Center for Human Interface Nanotechnology (HINT) Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea E-mail: kimsw1@skku.edu H.-J. Yoon, Dr. M. K. Gupta, J. H. Lee, W. Seung, H. Ryu, Prof. S.-W. Kim School of Advanced Materials Science and Engineering Sungkyunkwan University (SKKU) Suwon 440-746 , Republic of Korea

1. Introduction

The emergence of small-scale electronic devices with unprec-edented multifunctionalities essentially requires portable, light-weight, and sustainable power sources. Harvesting ambient energy sources including solar, thermal, and mechanical energies from nature has received increasing interest for self-powering electronics and portable devices with low power con-sumption. [ 1–4 ] To harvest mechanical energy wasted in nature, piezoelectric nanogenerators (PNGs) have been intensively developed to harvest mechanical energy with variable ampli-tude and frequency (such as air fl ow, raindrop, sound, human motion, and ocean waves). [ 5–8 ] To effectively convert various kinds of mechanical energies into electricity for the realization of the fully independent, sustainable, and wireless operation

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micropatterned PNGs were measured and compared to the output of nonpatterned P(VDF-TrFE)-based PNG under the same vertical compressive force. A photolithography process was used to develop different types of microstructure in P(VDF-TrFE) fi lm. The β phase of micropatterned P(VDF-TrFE) was confi rmed by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy studies. It was found the output voltage and current of microstructured P(VDF-TrFE)-based PNGs showed high value, with nearly fi ve times enhance-ment in the output voltage and current compared to the fl at fi lm-based PNG. COMSOL simulation software was employed to study the power enhancement in term of pressure created under micropatterned and nonpatterned of P(VDF-TrFE) fi lm. In addition, we successfully developed a self-powered pres-sure sensor and demonstrated that minute pressure devia-tion caused by water droplet and wind fl ow on micropatterned PNGs could be detected.

2. Results and Discussion

Schematic illustrations of the fl at, trigonal line-shaped, and pyramid-shaped micropattern structured P(VDF-TrFE) layers of the Indium tin oxide (ITO)-coated polyethylene naphtha-late (PEN) substrate are shown in Figure 1 a–c, respectively. To compare the morphology of the fl at and patterned P(VDF-TrFE) fi lms, fi eld emission scanning electron microscopy (FE-SEM) images were taken (Figure 1 d–f). Cross-sectional FE-SEM

images of P(VDF-TrFE) are shown in the inset of Figure 1 d–f. The thickness was confi rmed to be round 15 µm for all three P(VDF-TrFE) fi lms. The details of the fabrication process of both microstructured and the fl at P(VDF-TrFE)-based PNGs are illustrated in Figure S1, Supporting Information. Figure 2 a pre-sents the XRD spectra of P(VDF-TrFE) fi lm recorded at room temperature. The diffraction peaks at 2 θ values of 19.9° were assigned to the refl ections of the (110) and (200) orientation planes related to the polar β phase of P(VDF-TrFE). The strong diffraction peak indicated the high degree of crystallinity of the β phase in P(VDF-TrFE). An FT-IR spectroscopy spectrum of the P(VDF-TrFE) thin fi lm (Figure 2 b) illustrates three β phase-associated intense bands, with 1288 and 850 cm −1 belonging to the CF 2 symmetric stretching with the dipole moments parallel to the polar b axis. The 1400 cm −1 band was assigned to the CH 2 wagging vibration with the dipole moment along the c axis. [ 28 ]

We subsequently fabricated fl exible PNGs based on the fl at and micropatterned P(VDF-TrFE) thin fi lms. The silver (Ag) thin fi lm as a top electrode was deposited on the surface of polymers fi lm. An electric fi eld poling with the same strength was applied on all devices before measuring electric signals. We measured the output voltage and current from the fl at and micropatterned P(VDF-TrFE) fi lm-based PNGs by applying a pushing force to the top of the PNGs in the vertical direc-tion using a mechanical force stimulator. Piezoelectric output voltage with an average value of 0.4, 2.0, and 2.9 V and piezo-electric output current with an average value of 100, 500, and 900 nA from the fl at, trigonal line-shaped, and pyramid-shaped

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Figure 1. Schematic diagrams of a) fl at, b) trigonal line-shaped, and c) pyramid-shaped P(VDF-TrFE)-based PNGs. d–f) Geometrical description through FE-SEM is provided respectively. Insets in FE-SEM images correspond to cross-sectional images of each P(VDF-TrFE)-based NGs (scale bars suggested for each description is 10 µm).

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P(VDF-TrFE) thin fi lm-based PNGs were obtained under the vertical compressive force of 0.15 kgf as shown in Figure 3 a,b. The output voltage and current of microstructured P(VDF-TrFE)-based PNGs showed high value nearly fi ve times of enhancement was observed in the output voltage and current compared to the fl at fi lm-based PNG. The output power varia-tion of the pyramid-shaped micropatterned P(VDF-TrFE)-based PNG was investigated depending upon the loading resistance under the normal compressive force of 0.15 kgf. Figure 3 c shows the load resistance dependence of both output voltage and the current with the resistance ranging from 10 4 to 10 9 Ω. The output current decreases as load resistance increases due to the ohmic loss, while the output voltage tends to increase. The maximum power was obtained at a load resistance of 10 6 Ω, corresponding to a peak power of about 2.8 µW (Figure 3 d).

To verify that the measured signals were generated by the PNGs device rather than the measurement system, we conducted switching-polarity tests. When the current meter was connected reversely, the pulses were also reversed (see Figure S2, Supporting Information).

The working mechanism of P(VDF-TrFE)-based PNGs can be understood in terms of piezoelectric polarization under mechanical deformation (see Figure S3, Supporting Information). When we applied the electric fi eld poling on the polymers, the electric dipoles into the P(VDF-TrFE)

thin fi lm aligned symmetrically (e.g., from top to bottom elec-trode) due to its ferroelectric property (indicated by arrows). If the P(VDF-TrFE)-based PNGs was subjected to a vertical compressive strain, a positive piezoelectric potential was developed at the bottom side of the P(VDF-TrFE). An oppo-site negative potential was set up at the top surface of the P(VDF-TrFE). A potential difference was developed between the bottom and top electrodes. As a result, the piezoelectric potential-induced electrons started to fl ow from the Ag elec-trode to the ITO electrode through an external load resistor, giving an electric signal under forward connection. As the strain was released, the piezoelectric potential immediately vanished and the electrons accumulated near the ITO elec-trode fl ow back through the external circuit to the Ag elec-trode. An electric signal was observed in opposite direction.

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Figure 2. Characterization of β phase of P(VDF-TrFE). a) XRD and b) FT-IR spectroscopy results of the P(VDF-TrFE) thin fi lm.

Figure 3. Experimentally observed a) output voltage and b) current from the fl at, trigonal line-shaped, and pyramid-shaped P(VDF-TrFE)-based PNGs under the vertically applied constant compressive force of 0.15 kgf. c) Dependence of the output voltage and current as a function of the external load resistance and d) corresponding peak power output at 0.15 kgf.

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As a result, continuous peak-to-peak electric signal was gen-erated under application of stress and release of force.

The above result demonstrated that the output voltage and cur-rent in case of micropatterned P(VDF-TrFE)-based PNGs were signifi cantly larger compared to the fl at fi lm-based PNG. This enhancement could be attributed to different strain caused by different degree of sensitivity from the trigonal line-shaped, pyr-amid-shaped micropatterned as compared to the fl at P(VDF-TrFE) fi lm under application of same 1 kgf. It was clear that, although the applied force is same in all cases, due to morphological differ-ence, the strain development was different. The pyramid-shaped micropatterned P(VDF-TrFE)-based PNG exhibited high output performance than that of trigonal line-shaped micropatterned P(VDF-TrFE)-based PNG under same force. To confi rm the sta-bility and better performance of pyramid-shaped micropatterned P(VDF-TrFE) over others, the output voltage and current were measured from the fl at, trigonal line-shaped, pyramid-shaped P(VDF-TrFE)-based PNGs at different mechanical force.

The magnitude of the output voltage and current generated by the fl at, trigonal line-shaped, and pyramid-shaped micropat-terned P(VDF-TrFE)-based PNGs under application of force in range from 0.15 to 1.65 kgf are shown in Figure 4 a,b. It was found that the output voltage and current increased with the increase of force in all NGs. The output voltage and cur-rent from pyramid-shaped PNG showed high performance under application of all forces. The output voltage and current reached up to the value of 4.4 V and 3.4 µA from pyramid-shaped P(VDF-TrFE)-based PNG at 1.65 kgf. This results confi rmed that pyramid-shaped mor-phology was the best for high output perfor-mance from P(VDF-TrFE)-based PNGs under vertically applied external force. Mechanical durability test results ( Figure 5 a,b) under different pressure of 0.15 and 1.05 kgf con-fi rmed the prominent mechanical dura-bility of the micropatterned P(VDF-TrFE)-based PNGs. A very stable output voltage was obtained over 5000 cycles in all cases (Figure S5, Supporting Information).

To theoretically investigate the differ-ence in the power output from the fl at and micropatterned P(VDF-TrFE)-based PNGs,

COMSOL simulation investigation was per-formed. The relation of piezoelectricity can be explained by combination of the electrical behavior of the material (1) and Hooke’s Law (2) as given in following equations:

D Eε= (1)

S sT= (2)

where D is the electric charge density dis-placement, ε is permittivity, E is electric fi eld strength, S is strain, s is compliance, and T is applied stress. To a good approximation, those interaction can be described by linear relations as shown

dS s T EE= + (3)

dD T ETε= + (4)

The above equation confi rmed that strain variations that gives different piezoelectric output. [ 30,32 ] Using the above con-cept, strain distributions in the fl at, trigonal line-shaped, and pyramid-shaped morphology under same constant force were examined. Color-coded results are shown in Figure 6 . The simulation results depicted that pyramid-shaped structure had strong variation in the stain along the thickness of thin fi lm compared to the fl at and trigonal line-shaped micropatterned thin fi lms. Therefore, higher strain produced higher piezoelec-tric signal, giving higher output voltage and current for the pyr-amid-shaped microstructured P(VDF-TrFE)-based PNGs.

It was noted that micropatterned P(VDF-TrFE) exhibiting high sensitivity in response of even small force could be uti-lized as self-powered pressure sensors. One of its practical applications is a mechanical stimuli sensor system. A novel P(VDF-TrFE)-based piezoelectric climate sensing system was developed ( Figure 7 ). A virtual sensor module adhered on typical window is recognizable in Figure 7 a. Enlarged digital image was provided as an inset. The P(VDF-TrFE)-based pres-sure detector could alarm a sudden change in weather, such as

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Figure 4. a) Output voltage and b) current from the fl at, trigonal line-shaped, and pyramid-shaped P(VDF-TrFE)-based PNGs under various working forces vertically applied onto substrate.

Figure 5. Examination of mechanical durability for different (the fl at, trigonal line-shaped, and pyramid-shaped) P(VDF-TrFE) PNGs; a,b) Stable output voltage generation from all P(VDF-TrFE) PNGs with different applied force of 0.15 and 1.05 kgf, respectively.

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raining or wind velocity assumed to be as external force. That is, whatever certain atmospheric changes within the perfor-mance induced by devices, it can be operated by itself without any supply of electricity. Maximum transformed output from a given input is the most preferable way. Figure 7 b,c shows comparative output voltage data for all types of pressure sensor including the fl at, trigonal line-shaped, and pyramid-shaped patterned under raining circumstance and windy case. Detailed experimental set-up for both instances are described in Figure S6 (Supporting Information).

With ≈80 mL of water droplet freely falling 142 mm, enhanced amount of stress was applied on both patterned

devices that have an increase in output voltage. The actual water droplet on device is shown as inset. Its diameter was determined through an engineering metal ruler. It was ≈7 mm when it constantly stayed on the device. Another practical appli-cation is wind sensor. This innovative system not only enables us to harvest electrical power identical to commercial wind turbines but also provides information of wind strength as a detectable mechanical force. Relatively low speed wind that can deliver small amount of energy to the device can also be recognized as shown in Figure 7 c. We found unprecedented sensitivity in an extremely low wind pressure derived from 0.5 m s −1 of wind velocity, with patterned surfaces showing

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Figure 6. Simulation results showing structured surfaces such as trigonal line-shaped and pyramid-shaped P(VDF-TrFE)-based PNGs. 1 × 1 dimen-sional devices were modeled employing COMSOL. Total height of all devices was set as 15 µm with applied vertical compression of 0.1 kgf. Derived strain from given stress is shown in a). Further study about output voltage induced by following strain is demonstrated in (b).

Figure 7. Highly sensitive pressure-sensing devices derived from micropatterned P(VDF-TrFE). a) Climate sensor is situated onto the window outward. Inset: Enlarged illustration of the rain sensor; b) Output voltage generated from all types of sensors with respect to patterns on surface as dropping water droplet. Inset: Water droplets staying at the sensor (scale bar: 10 mm); c) Output voltage induced by sensors regarding the patterns on surface as stirring up the wind using an air gun. Inset: Wind speed was recorded by utilizing an anemometer.

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almost fi ve orders of magnitude higher than fl at surface. With an air generation system, we could make an artifi cial windy condition experimentally and measure its speed utilizing com-mercial anemometer as given in inset of Figure 7 c. In all cases, micropatterned surface, especially the pyramid-shaped one, has the highest sensitivity regarding the external pressure, which clearly tells us that structured surface is prominently suitable for detecting minuscule stress. It is quite effective for micro-structured surface to be used to respond rapidly to external stimuli. This can be used in alarm system such as for rain starting or wind rising without any electrical energy storage such as battery.

3. Conclusion

In summary, we successfully enhanced the electric output power from P(VDF-TrFE)-based PNGs via a simple micropattering pro-cess. Two different types of morphologies (trigonal line-shaped and pyramid-shaped) were developed on P(VDF-TrFE) polymer using photolithography and their PNGs were fabricated. The output voltage and current were measured from the fl at and micropatterned piezoelectric P(VDF-TrFE) polymers under vertical compressive force. Higher output voltage and current with average values of 4.4 V and 3.4 µA were obtained from the micropatterned P(VDF-TrFE)-based PNG under vertical mechanical compressive force compared to the fl at fi lm-based PNG (1 V and 100 nA). The pyramid-shaped micropatterned P(VDF-TrFE)-based PNG exhibited larger power output than the trigonal line-shaped micropatterned P(VDF-TrFE)-based PNG. The output performance was increased with the increase of compressive force. Very stable electric power generation perfor-mance over 5000 cycles was obtained. The enhancement in the output power was discussed using COMSOL simulation in term of different strain created in the fl at and micropatterned piezo-electric fi lm under the same constant force. Due to its ultrahigh sensitivity, the micropatterned PNGs were also demonstrated as a self-powered pressure sensor. Our demonstration of power enhancement in micropatterned P(VDF-TrFE)-based PNGs may greatly contribute to the development of self- powering small scale devices or systems with low operating power consumption.

4. Experimental Section Synthesis of Micropatterned P(VDF-TrFE) and Fabrication of PNGs :

P(VDF-TrFE) solution was prepared by dissolving P(VDF-TrFE) copolymer powder in N , N -dimethylformamide (DMF) with stirring for 12 h at room temperature. Solution of P(VDF-TrFE) (≈20 wt%) was spin-coated on fl exible ITO-coated PEN substrate at 3000 rpm for 60 s, followed by drying at 80 °C to remove the DMF solvent. For micropattering, trigonal line-shaped, and pyramid-shaped patterned Si wafer master mold was fabricated from (100) Si wafers with a 300 nm thermally grown oxide by simple photolithography and etching process to release the mold. Micropatterned layer was annealed at 140 °C for 2 h and then naturally cooled down to room temperature gradually in order to enhance the crystallinity of the β phase. For device fabrication, Ag electrode of average thickness of 200 nm was deposited by using a thermal evaporation method. The electrical poling process was carried out by applying an external electric fi eld of 80 MV m −1 for 2 h to align the electric dipoles in a single direction.

Characterization and Measurements : FE–SEM characterization was performed for the morphological investigation of the fl at, trigonal line-shaped, and pyramid-shaped micropatterned P(VDF-TrFE) fi lms. The synthesized piezoelectric polymers underwent structural and crystallographic characterizations using XRD (Bruker D8 DISCOVER) with Cu KR radiation ( λ = 1.54 Å). FT-IR measurements were used for the structural investigation of the β phase of P(VDF-TrFE) thin fi lms. A pushing tester (Labworks Inc., Model No. ET-126-4) was utilized to create strain in P(VDF-TrFE). A Tektronix DPO 3052 Digital Phosphor Oscilloscope and low-noise current preamplifi er (Model No. SR570, Stanford Research Systems, Inc.) were used to measure electrical signal.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements J.-H.L. and H.-J.Y. contributed equally to this work. This work was fi nancially supported by Basic Science Research Program (2012R1A2A1A01002787, 2009-0083540) and the Center for Advanced Soft-Electronics as Global Frontier Project (2013M3A6A5073177) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT and Future Planning.

Received: March 3, 2015 Revised: March 20, 2015

Published online: April 15, 2015

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